bugfix: setup-env.sh detects bash even when bash is run as sh (#11994)
bugfix: setup-env.sh detects bash even when bash is run as sh (#11994)

- `setup-env.sh` was not properly detecting a bash shell when bash was run
  as /bin/sh.

- Detection routine now always reports bash when bash is run as sh, and
  no longer parses the path to the executable indicated in `$BASH`.
iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack